Monitor Java Garbage Collection using jstat

After understanding the garbage collection concept in JAVA, this article lists the "jstat" commands&options to monitor JAVA GC.

-gc

It shows the current size for each heap area and its current usage (Eden, survivor, old, etc.), total number of GC performed, and the accumulated time for GC operations.

[root@admin]# jps
2304 ResourceManager
[root@admin]# jstat -gc 2304 1000
 S0C    S1C    S0U    S1U      EC       EU        OC         OU       PC     PU    YGC     YGCT    FGC    FGCT     GCT
8064.0 8064.0  0.0   388.5  65088.0  61409.2   163520.0    8074.1   31616.0 31553.6      9    0.149   0      0.000    0.149
8064.0 8064.0  0.0   388.5  65088.0  61409.3   163520.0    8074.1   31616.0 31553.6      9    0.149   0      0.000    0.149

Above commands firstly find out the vmid of "Resource Manager", and check the overall GC situation every 1000 ms(1 second).
Below are the description for each column.

Column	Description
S0C	Current size of Survivor0 area in KB
S1C	Current size of Survivor1 area in KB
S0U	Used Survivor0 area in KB
S1U	Used Survivor1 area in KB
EC	Current size of Eden area in KB
EU	Used Eden area in KB
OC	Current size of old area in KB
OU	Used old area in KB
PC	Current size of permanent area in KB
PU	Used permanent area in KB
YGC	The number of GC event occurred in young area
YGCT	The accumulated time for GC operations for Yong area
FGC	The number of full GC event occurred
FGCT	The accumulated time for full GC operations
GCT	The total accumulated time for GC operations

Above example output shows: YGC=9, YGCT=0.149(seconds), which means each GC in Young area took 0.149/9=0.0166(seconds).

-gcutil

Shows the usage for each heap area in percentage. Also shows the total number of GC performed and the accumulated time for GC operations.

[root@admin]# jstat -gcutil 2304 1000
  S0     S1     E      O      P     YGC     YGCT    FGC    FGCT     GCT
  5.00   0.00   5.87   4.94  99.84     12    0.164     0    0.000    0.164
  5.00   0.00   5.87   4.94  99.84     12    0.164     0    0.000    0.164

-gccause

It shows the "information provided by -gcutil" + reason for the last GC and the reason for the current GC.

[root@admin ~]# jstat -gccause 2304 1000
  S0     S1     E      O      P     YGC     YGCT    FGC    FGCT     GCT    LGCC                 GCC
  5.00   0.00  32.97   4.94  99.84     12    0.164     0    0.000    0.164 Allocation Failure   No GC
  5.00   0.00  33.29   4.94  99.84     12    0.164     0    0.000    0.164 Allocation Failure   No GC

Column	Description
LGCC	The cause for the last GC occurrence
GCC	The cause for the current GC occurrence

-gcnew

Shows the GC performance data for the new area.

[root@admin ~]# jstat -gcnew 2304 1000
 S0C    S1C    S0U    S1U   TT MTT  DSS      EC       EU     YGC     YGCT
8064.0 8064.0  403.4    0.0  6   6 4032.0  65088.0  30030.4     12    0.164
8064.0 8064.0  403.4    0.0  6   6 4032.0  65088.0  30030.4     12    0.164

Column	Description
TT	Tenuring threshold. If copied this amount of times in young area (S0 ->S1, S1->S0), they are then moved to old area.
MTT	Maximum Tenuring threshold.
DSS	Desired survivor size in KB.

-gcold

Shows the GC performance data for the old area.

[root@admin ~]# jstat -gcold 2304 1000
   PC       PU        OC          OU       YGC    FGC    FGCT     GCT
 31616.0  31565.9    163520.0      8074.1     12     0    0.000    0.164
 31616.0  31565.9    163520.0      8074.1     12     0    0.000    0.164

-gcpermcapacity

Shows statistics for the permanent area.

[root@admin ~]# jstat -gcpermcapacity 2304 1000
  PGCMN      PGCMX       PGC         PC      YGC   FGC    FGCT     GCT
   21248.0    83968.0    31616.0    31616.0    12     0    0.000    0.164
   21248.0    83968.0    31616.0    31616.0    12     0    0.000    0.164

Column	Description
PGCMN	The minimum size of permanent area in KB.
PGCMX	The maximum size of permanent area in KB.
PGC	Current size of permanent generation area in KB.

-printcompilation

HotSpot Compiler Method Statistics.

[root@admin ~]# jstat -printcompilation 2304 1000
Compiled  Size  Type Method
     568    178    1 org/mortbay/jetty/HttpFields destroy
     568    178    1 org/mortbay/jetty/HttpFields destroy

Column	Description
Compiled	Number of compilation tasks performed by the most recently compiled method.
Size	Number of bytes of bytecode of the most recently compiled method.
Type	Compilation type of the most recently compiled method.
Method	Class name and method name identifying the most recently compiled method. Class name uses "/" instead of "." as namespace separator. Method name is the method within the given class. The format for these two fields is consistent with the HotSpot - XX:+PrintComplation option.

-class

Show Class Loader Statistics.This is to detect ClassLoader leaks.

[root@admin ~]# jstat -class 2304 1000
Loaded  Bytes  Unloaded  Bytes     Time
  5798 11127.9        0     0.0       4.46
  5798 11127.9        0     0.0       4.46

Column	Description
Loaded	Number of classes loaded.
Bytes	Number of Kbytes loaded.
Unloaded	Number of classes unloaded.
Bytes	Number of Kbytes unloaded.
Time	Time spent performing class load and unload operations.